This was posted on a Bernina group site. Probably good advice for other sewing machine brands as well. Wish I could give more credit to the poster but don't have more information:
Turn your machines on every now and again - Bernina advisory
ietzman Message 1 of 4 , Jun 18, 2014
I don't post much here but I am a constant lurker.
This was posted on one of my FB groups by a well respected Bernina tech.
"Bernina recently issued a technical advisory note that backs-up a post I put on FB a while back in relation to the 'regular' use of sewing machines. I previously advised that you should make a point of switching-on your machine every so-often, even if you dont want to use it, and leaving it on for an hour-or-so. This mainly applies to older electronic machines from the Activa/Virtuosa/Aurora/Artista vintages, but is equally good advice on later models. The Bernina tech advice relates to failure of electrolytic capacitors on power supply boards if unused for a long time - this applies to stock held on shelves but, equally, to machines too. I have also seen this problem on motherboard/control boards, which are much more expensive and particularly with a variety of electrolytic capacitors called tantalum. This problem is NOT unique to Bernina and is a technology problem rather than a manufacturing quality problem. This is sometimes experienced when you put away a piece of electronic equipment such as a video camera or recorder or some-such and dont use it for years and find its faulty when you get it out to use again. Modern capacitors are much better in this regard. So, if you're an 'occasional' user of your machine, of whatever make, my advice is, every 6 months, get it out, switch it on and just leave it for an hour-or-so powered-up and this should (and I emphasise 'should' !) protect it against failure of these components and save you the price of a potentially costly repair. This is especially important for the many of you who cant part with their old machines, but never really seem to get-around to using them !!"